1. Field of the Invention
The present invention relates to a semiconductor device, for example, a NAND type flash memory, and in particular, to an auto trimming function for automatically adjusting a voltage generated inside the semiconductor device.
2. Description of the Related Art
A continuous reduction in the size of semiconductors has made the reliability of transistors more and more important. To improve the reliability of transistors, it is necessary to minimize voltages applied to the transistors. On the other hand, increasing the operation speeds of the transistors requires as high voltages as possible to be applied to them. In particular, the internal voltage of the semiconductor device is often set higher than a power voltage Vcc in order meet the need for fast operations. Thus, techniques for maintaining the reliability of the transistors are essential.
A variation in internal voltage is a factor preventing the achievement of both increased operation speed and improved reliability. An internal voltage generation circuit is commonly an analog circuit. Thus, the characteristics of the circuit vary sensitively with process parameters such as the threshold voltage Vth, wiring resistance Rs, and oxidized film thickness. Moreover, an internal voltage generation circuit has current mirror circuits. The characteristics of the circuit thus vary depending on the fine pattern shape of the current mirror circuit. The variation in internal voltage tends to be accelerated by the recent reduction in the size and power voltage Vcc of the semiconductor device.
A trimming function is a method for reducing a variation in internal voltage. The trimming function sets an internal voltage at a target value by, for example, finely adjusting the resistance value of a voltage dividing resistor provided at an output stage of the internal voltage generation circuit. In recent years, auto trimming has been proposed which compares an internal voltage generated inside a semiconductor device with an externally supplied target voltage to trim the internal voltage in a self-aligning manner on the basis of the comparison (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-229697).
This auto trimming is generally executed during a die sort test. Trimming data resulting from the auto trimming is stored in a register or an electric fuse. In other words, if after the auto trimming, evaluations are continued without turning off a power supply voltage, the trimming data is stored in the register circuit. If the trimming data is permanently used in products or the like, it is stored in the electric fuse.
For example, NAND type flash memories require high internal voltages to write, verify, or erase data. In particular, if after a write verify operation, a rewrite operation is performed by gradually raising a write voltage or if a plurality of data are stored in one memory cell as in a multivalue memory, then the threshold voltage of each memory cell needs to be precisely controlled. This also requires the internal voltage to be precisely controlled. Further, for example, dynamic RAMs require an accurate internal voltage in order to achieve both increased operation speed and improved reliability as described above. Therefore, generation of an accurate internal voltage have been desired.